Interaction of slip‐ and flame‐spray coated carbon‐bonded alumina filters with steel melts

Ceramic foam filters play an essential role in the quest for cleanliness of cast steel parts as they facilitate turbulence reduction during mold filling as well as removal of nonmetallic inclusions. A coating on these filters is able to increase their strength and filtration efficiency by improving the adhesion of inclusions to the filter strands. In this study, Al₂O₃‐C filters were coated with an alumina slip via slip and flame spraying. The phase composition and the microstructure of the coatings were investigated before and after immersion into molten steel contained in a metal casting simulator. After contact with molten steel, Al₂O₃‐C reference filter shows intense decarburization which often influence the quality of cast steel parts due to formation of gas bubbles. Slip‐sprayed alumina coatings on such a filter promote the deposition of inclusions due to formation of a vitreous alumina layer but will also cause gas bubble formation as they exhibit a high porosity. Flame‐spray coatings have low porosity and hence, prevent formation of gas bubbles. Furthermore, they showed the highest reactivity toward the steel melt and hence, are recommended for filtration of cast products with a high demand on cleanliness.     

Effect of α-Al2O3 content on microstructures,mechanical properties and purification efficiency on molten steel of MgO-based filters

In this study, five sets of MgO-based ceramic filters were prepared using porous MgO powder and α-Al₂O₃ micro-powder as raw materials. The effect of α-Al₂O₃ content on the microstructures, mechanical properties and purification efficiency of MgO-based filters were investigated via SEM, EDS, and immersion test in molten steel, etc. The results indicate that the reactive sintering between α-Al₂O₃ and MgO promoted the formation and growth of neck connections of magnesium aluminate spinel between grains, increased the density of the filter skeleton, and decreased the pore size inside the skeleton. The higher density and smaller pore size of the skeleton, as well as the neck connection of spinel and crack deflection effect caused by magnesium aluminate spinel, comprehensively improved the strength and thermal shock resistance of MgO-based ceramic filters. Furthermore, the results from the immersion test in molten steel revealed that an appropriate amount of high-temperature liquid phase, smaller pore size, and in-situ generated spinel were beneficial for the filters to adsorb dissolved aluminum, dissolved oxygen, alumina inclusions, and secondary spinel inclusions in the steel. The filter with 20 wt% α-Al₂O₃ had the best overall performance with a compressive strength of 2.35 MPa, a compressive strength after thermal shock test of 1.91 MPa and an efficiency of 77.3% in reducing the total oxygen content of the steel.     

Innovative carbon-bonded filters based on a new environmental-friendly binder system for steel melt filtration

New carbon-bonded alumina filters for steel melt filtration were developed. The carbonaceous matrix was based on a new, environmental friendly binder system based on lactose and tannin. The filter preparation was analogous to the production of conventional foam filters according to the Schwartzwalder process. The processing as well as the rheology of the slurries was investigated. An addition of n-Si increased the carbon yield and the cold crushing strength (CCS) of the samples. Higher values of CCS were obtained after coating of the filters with alumina. The material was characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The applicability of these new filters was assessed in impingement tests with a steel melt, in which three out of four recipes survived the thermal shock.     

Towards a new generation of environmentally-friendly ceramic foam filters: contribution of graphene nanoadditives in calcium aluminate-rich coatings

In this contribution, the widely used non-ecological production process of ceramic foam filters was modified into an environmentally friendly procedure, in which no hazardous materials are involved. Firstly, a large number of carbon-bonded ceramic foam filters were prepared by this approach. In the next step, a thin coating consisting of graphene-doped, carbon-bonded calcium aluminate was applied over these filters. Reference samples as well as coated samples were analyzed in detail with a focus on the phase composition, microstructural analysis, and mechanical as well as thermal properties. The results confirmed that the application of the developed coating improved the cold crushing strength of the ceramic foam filters. In addition, by application of graphene nanoadditives, it is also possible to vary the surface microroughness which will subsequently enable improved efficiency of inclusions elimination from metal melts during the filtration process.     

Alumina coatings on carbon bonded alumina nozzles for active filtration of steel melts

Slip casted, carbon bonded nozzles with and without alumina based active coatings have been simultaneously tested in a novel casting simulator according to their clogging behavior against endogenous as well as exogenous non-metallic inclusions. Computer tomography images as well as SEM micrographs support the investigation of the clogging areas. The nozzle with the alumina active coating leads to increased clogging phenomena based on endogenous and exogenous inclusions as well as steel clogged particles. Correlating the clogging to a type of improved “filtration efficiency”, a higher filtration of steel melts is expected with the application of alumina active coatings on carbon bonded filters.     

Steel contamination and growths on batching devices for continuous casting

Conclusions Delivery of metal with a closed jet facilitates a big reduction in the quantity of nonmetallic inclusions in the continuously cast ingot.By examining the kinetic rules governing the reaction of floating inclusions with circulation currents we established the optimum height of the molten steel level in the intermediate ladle as being equal to 600–650 mm.During the tightening of the nozzle, compounds enriched with alumina (highly dispersed refractory crystalline compounds of the corundum, mullite, and spinel type) are deposited on the walls of the channel.The higher the metal's concentration of inclusions containing large quantities of Al₂O₃, then the more intense is their bonding with the refractory, leading to tightening up and growth of the nozzle.Translated from Ogneupory, No. 8, pp. 22–27, August, 1970.     

Clean Steel Technologies Based on Interactions of Refractory Filtering Materials with Steel Melt

New carbon bonded filters with "active and reactive coatings " for higher filtration efficiency of alumina based inclusions as well as nano-engineered filters with nano-scaled additives are explored with the aid of impingement tests and are evaluated according to their cold crushing strengths at room temperature. The combination of carbon nanotubes and alumina nanosheets additives leads to in situ formation of Al3 CON. Both the nanoscaled additives as well as the extra alumina "active"coating lead to improved mechanical performance of the carbon bonded filters and open the horizon for filter macrostructures with higher filtration capacities in means of bigger dimensions. In a further step MWCNT( multi walled carbon nanotubes) in combination with a synthetic pitch have been used as a functional coating on the surface of alumina carbon bonded filters. These filters have then been evaluated also with uncoated filters in a special casting simulator and the interactions between steel and filtering material have been investigated.     

Nano-functionalization of carbon-bonded alumina using graphene oxide and MWCNTs

This paper focused on the nano-functionalization of carbon-bonded alumina using graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs). GO was prepared according to the modified Tour's method. The rheology of suspensions containing GO, CNTs and both substances were analyzed and discussed. Xanthan proved to be a suitable stabilizer for the three systems. The spraying process of the suspensions was investigated with the aid of a high-speed camera. Al₂O₃-C filters and flat samples were spray coated, for investigations in contact with steel. The ceramic foam filters as well as the starting suspensions were analyzed by several analytic techniques to investigate the microstructure and other properties. Moreover, the hot stage microscope was used to study the steel/coating wetting behavior during operation at high temperatures. The results showed that the formulation containing both GO and CNTs delivered the best performance in contact with the melt. Similar coatings based on these nano-sized materials may offer an innovative route to improve purification of steel melts by filtration.     

Mechanical and physical characterization of Al2O3–C foam filters produced by distinct processing routes: The importance of the ceramic strut morphology

In this work the influence of the production route and filter dimensions on the mechanical properties of carbon-bonded alumina foam filters was investigated. Features like cold crushing strength and fracture behavior were analyzed and used to compare them. Microfocus computer tomography (μCT) was applied to characterize the geometry of the filters, providing a deep correlation with their fracture behavior. Furthermore, computer-generated filter geometries were used as reference. The results indicated that the centrifugation process is better suited for the production of carbon-bonded alumina filters, and is even more effective when the foam dimensions are increased in the range of those analyzed in this research. Finite element simulations showed the influence of the relative density and strut tapering on the cold crushing strength of filters, providing a correlation between filter structure and its failure mechanism. The attained results provided further insights towards the production of inclusions-free metal parts.     

模铸用高铝质流钢砖的显微结构分析

选取市场上模铸用高铝质流钢砖与用后残砖作为研究对象,通过检测不同高铝质流钢砖的理化性能,对比其使用前后的显微结构变化,进一步探讨模铸过程中高铝质流钢砖的显微结构对钢铁产品质量的影响。结果表明:由于显气孔率较大、结构比较疏松、烧结不致密等特性,高铝质流钢砖在经受钢水冲刷时,钢水容易渗透至高铝质流钢砖内部,加速其损毁;其次,高铝质流钢砖与钢水发生物理化学反应后在其表面生成低熔点物,使钢水中产生夹杂物,影响钢铁产品的质量。     

Preparation of electrolytic ceramic films on stainless steel conversion coatings

An electrolytic method for the synthesis of an alumina barrier on stainless steel with strong interfacial bonding is described for a Fe-17%Cr alloy. The deposit was laid down electrochemically after a specific conversion treatment by chemical oxidation of the substrate in acid solution. The conversion coating was very porous and had excellent adhesion at the substrate interface. Alumina was obtained by thermal dehydration of aluminium hydroxide deposited from an aqueous solution of an aluminium salt, according to a two-step mechanism: generation of hydroxyl ions at the cathodic substrate by reduction of H₂O or dissolved oxygen and a precipitation reaction forming aluminium hydroxide. Thermal treatment induced interfacial reactions between aluminium oxide and conversion coating compounds which led to spinel formation beneath the superficial alumina layer. The coating presented chemical composition gradients suitable for strong adhesion. Thermal oxidation resistance was studied in air at 1000° C.     

钢水的钙处理及含尖晶石耐火材料的开发利用

钙处理铜水中［Ｃａ］在钢水、耐火材料界面上与材料中的ＳｉＯ２进行化学反应，生成的ＣａＯ又被吸附在耐火材料表面上。因材料材质不同，表面所形成的矿物不同及液相比的差异，导致抗侵蚀能力的不同。含尖晶石耐火材料则可以提高这种抗侵蚀能力。利用对ＭｇＯＣａＯ－Ａｌ２Ｏ３－ＳｉＯ２四元系研究结果，指出尖晶石耐火材料的性能除了与Ａｌ２Ｏ３、ＭｇＯ、尖晶石含量有关外，还与其它矿物组成有密切关系，并指出最佳的矿物组成范围。     

Alumina-spinel castable for steel ladles: An overview

Alumina-spinel castable has excellent refractory properties, including high volume and thermal stability, good hot strength, and excellent resistance against corrosion, erosion, and thermal spalling. Excellent refractory properties have made it a preferred lining for steel ladles, reducing the erosion and corrosion of molten metal and slag and improving the ladle life and efficiency for decades. These castables are also significant for the replacement of chrome-bearing refractories that cause several health hazards. The current review paper covers preformed and in situ spinel containing alumina castables. It focuses on the importance of these castables in the sidewall and bottom portion of the steel ladle in detail, the effect of spinel and MgO content, and the factors affecting castable properties. The review also covers the final properties of castable and the application areas of alumina-spinel castable in steel ladle, purging plugs, and RH-degasser.     

Preparation of high performance MgO ceramic filter and its interaction with molten steel: Effect of porous MgO powder

In this study, two kinds of MgO ceramic filters were prepared by using porous magnesia powder and fused magnesia powder as raw material, respectively. The microstructures, mechanical properties and purification efficiency on the molten steel of two MgO ceramic filters were investigated by mercury porosimetry measurement, SEM, EDS and immersion test with molten steel, etc. The results demonstrate that compared with the filter FM prepared from the fused magnesia powder, the rheological property of the slurry of the filter PM was improved via the finer and uniformly distributed nano-sized Mg(OH)₂ particles generated by the hydration of the porous magnesia powder, which not only optimized the macropore structure but also promoted the growth of the sintering neck between the microparticles of the skeleton. Therefore, the MgO filter skeleton with a microporous structure was obtained, which elevated the strength and thermal shock resistance of the filter PM. According to the immersion tests with the molten steel, both filters significantly reduced the number of inclusions with size < 5 µm² in the molten steel as well as their total oxygen contents, and the filter PM had better purification efficiency than the filter FM. The reason for the better purification efficiency of the filter PM was related to its smaller micro-sized pores, larger surface roughness, and certain high-temperature liquid phase in their skeletons. In general, the porous magnesia powder resulted in higher compressive strength, better thermal shock resistance and superior purification efficiency on the molten steel of the filter compared with the fused magnesia powder.     

Dynamic interaction of refractory and molten steel: Effect of alumina-magnesia castables on alloy steel cleanness

High quality alloy steel is an important material needed for social and economic development. It is of great significance to major national projects and defence security. Refractories are used in the smelting process of steel; they are some of the main sources of impurities which have an important effect on the quality of steel. As alumina-magnesia refractories are the main lining materials used for steel refining, the influence of these refractories on the cleanliness of molten steel under dynamic smelting conditions has been studied. The size, quantity, composition, and structure evolution of inclusions in steel are analysed. The results show that after smelting, the content of alloy elements in the steel is stable, and that the total oxygen content and inclusions in the steel are increased by the corrosion of the alumina-magnesia castables. However, the maximum average particle size of the inclusions in the steel was limited to 20?µm, which did not cause large inclusions in the steel or seriously affect the quality of steel. During the dynamic melting process, because of the presence of Si and Mn in the alloy steel, the inclusions changed from homogeneous CaS wrapped Al₂O₃-MgO composite sphere to MnS wrapped egg-shaped structure. The alloy elements in steel were found to be beneficial, as they reduced the effect of alumina and magnesia inclusions on the quality of steel. The results indicate that it is feasible to smelt high quality alloy steel using alumina-magnesia carbon-free castable, and that it would be better to limit the refining time to 45?min during smelting.     

The corrosion resistance of microsilica-containing Al2O3–MgO and Al2O3–spinel castables

Microsilica addition in Al₂O₃–MgO and Al₂O₃–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA₆ was formed in the matrix, mainly for the Al₂O₃–spinel composition, which also favored the castable dissolution into the molten slag.     

富铝尖晶石对镁质耐火材料抗侵蚀性的影响

研究了富铝尖晶石对镁质耐火材料抗钢渣与抗钙处理钢侵蚀性的影响。结果表明 :随着富铝尖晶石加入量的增加 ,镁质耐火材料的抗钙处理钢和钢渣熔蚀性逐渐减弱 ,而抗钢渣渗透性逐渐增强 ;纯镁质和镁尖晶石质耐火材料在抗钢渣与抗钙处理钢侵蚀方面远远优于铝锆碳质材料     

Behavior of Chromium in the System MgA12O4– A12O3

Single crystals of chromium-doped magnesium aluminate spinels with varying amounts of excess alumina have been grown by the flame fusion process. Presumably these crystals contain stoichiometrically significant amounts of defects. When these chromium-doped spinels are annealed at temperatures below their melting point, the spinel structure cannot tolerate such a high concentration of defects and at least some of the excess alumina is exsolved. The chromium ion in magnesium aluminate spinel is pink, whereas in magnesium aluminate spinel containing excess alumina it is green. When exsolution of the alumina occurs during annealing, the aggregate changes to pink. The behavior of the chromium ion is monitored by observing the significant changes in the optical absorption as well as the emission or fluorescence spectra for both single crystals and polycrystalline materials in the system [MgA1₂O₄– A1₂O₃] :Cr.     

Development of novel carbon bonded filter compositions for steel melt filtration

Carbon-bonded alumina filters for steel melt filtration are increasingly used by steel foundries. Their potential regarding material characteristics and filtration efficiency has not been fully exploited yet. A new filter composition based on the synthetic coal tar pitch Carbores® P, has been explored. Its compatibility with a water-based dispersing system allows additions of carbon black, graphite, and additives to reach a residual carbon content of up to 30 wt.%. For determining physical properties and variables of the respective compositions, 10 pores per inch (ppi) foam ceramic filters were manufactured via the Schwartzwalder process. The influence of different binder contents on the processing properties and mechanical characteristics of the system was evaluated. Based on these studies, an increase in cold crushing strength of approx. 30% has been achieved in comparison to current state of the art materials.     

Improved thermal shock performance of Al2O3-C refractories due to nanoscaled additives

Carbon bonded alumina refractories with approximately 30 wt.-% residual carbon after coking are widely used as functional components such as submerged entry nozzles, monobloc stoppers and ladle shrouds in steel casting operations. Compositions with less residual carbon after coking based on nanoscaled magnesium aluminate spinel (MgAl₂O₄), alumina nanosheets (α-Al₂O₃) and carbon nanotubes (CNTs) either as single additives or combinations have been investigated according to their physical, mechanical and thermo-mechanical properties. The combination of nanoscaled powders based on carbon nanotubes and alumina nanosheets lead to superior thermal shock performance with approximately 30% less residual carbon in comparison to commercial available material compositions.